if (!elements.hasNext()) {
      return of();
    }
    E first = elements.next();
    if (!elements.hasNext()) {
      return of(first);
    }

    Set<E> delegate = Sets.newLinkedHashSet();
    delegate.add(checkNotNull(first));
    do {
      delegate.add(checkNotNull(elements.next()));
    } while (elements.hasNext());

    return unsafeDelegate(delegate);
  }

    List<Set<Optional<?>>> optionalSets = Lists.newArrayListWithExpectedSize(sets.length);
    for (Set<?> set : sets) {
      Set<Optional<?>> optionalSet =
          Sets.newLinkedHashSet(Iterables.transform(set, Optional::fromNullable));
      optionalSets.add(optionalSet);
    }
    Set<List<Optional<?>>> cartesianProduct = Sets.cartesianProduct(optionalSets);